Abstract: We report the measurement of the primordial D/H abundance ratio towards QSO
\object. The column density of the hydrogen in the $z \simeq 2.536$ Lyman limit
system is high, \lnhi $= 19.422 \pm 0.009$ \cmm, allowing for the deuterium to
be seen in 5 Lyman series transitions. The measured value of the D/H ratio
towards QSO \object is found to be D/H$ = 2.54 \pm 0.23 \times 10^{-5}$. The
metallicity of the system showing D/H is found to be $\simeq 0.01$ solar,
indicating that the measured D/H is the primordial D/H within the measurement
errors. The gas which shows D/H is neutral, unlike previous D/H systems which
were more highly ionized. Thus, the determination of the D/H ratio becomes more
secure since we are measuring it in different astrophysical environments, but
the error is larger because we now see more dispersion between measurements.
Combined with prior measurements of D/H, the best D/H ratio is now D/H$ = 3.0
\pm 0.4 \times 10^{-5}$, which is 10% lower than the previous value. The new
values for the baryon to photon ratio, and baryonic matter density derived from
D/H are $\eta = 5.6 \pm 0.5 \times 10^{-10} $ and \ob $=0.0205 \pm 0.0018$
respectively.

Comments:

Minor text and reference changes. To appear in the May 10, 2001 issue of the Astrophysical Journal